"The atmosphere can 'remember' its previous state by storing physical information in the form of water vapour," explains Anja Katzenberger, PIK researcher and lead author of the study. "In practical terms, this means that even though solar radiation increases or decreases with the seasons, the atmosphere doesn't always respond immediately. During spring, water vapour accumulates over days and weeks, forming a moisture reservoir that triggers the onset of monsoon rainfall in early summer and sustains it even as solar influx declines in autumn."
This research, published in the journal PNAS, reveals that the atmosphere's state is path-dependent, meaning its current condition is heavily influenced by past moisture levels. If the atmosphere is already moist, it tends to remain so, maintaining rainfall. Conversely, if it starts out dry, it is much harder to initiate the wet phase. This phenomenon, known as bistability, explains why monsoons can rapidly switch between dry and wet states depending on prior conditions.
"We've long known that large systems like oceans and ice sheets have some form of memory, but the atmosphere having a similar capability was previously unthinkable," says study co-author Anders Levermann, head of the Complexity Science department at PIK. "This atmospheric memory creates a switch-like behavior in monsoon rainfall, characterized by abrupt shifts rather than gradual transitions. What's particularly unique is that the monsoon system crosses this threshold every year, potentially allowing scientists to identify and monitor its tipping point with real-world data."
To unravel the mechanisms behind this bistability, the research team combined real-world observations from monsoon regions in India, China, and other key areas with advanced simulations using a high-resolution atmospheric general circulation model from Princeton University. By isolating atmospheric processes from slower Earth system components like oceans, the team demonstrated that monsoon rainfall can independently flip between dry and wet states. A critical finding was that when atmospheric water vapor reaches approximately 35 kilograms per square meter, the monsoon rapidly switches on, and when it drops below this level, it switches off.
This threshold-based response highlights the delicate balance within the monsoon system and raises concerns about the impact of climate change and pollution, which could disrupt this natural cycle. Levermann warns, "This would have dramatic consequences for billions of people in regions like India, Indonesia, Brazil, and China, where livelihoods depend heavily on monsoon rainfall. Disrupting this balance would not only destabilize our climate but also profoundly impact global societies."
Research Report:Monsoon Hysteresis reveals Atmospheric Memory
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Potsdam Institute for Climate Impact Research (PIK)
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